1. Field of the Invention
The invention relates to plastic implants with a circumferential channel for receiving a radiographic contrast wire.
2. Description of the Related Art
Plastic implants cannot be clearly imaged in X-rays because of the lack of contrast differentiation between them and the body tissue. In order to check the position of an inserted plastic implant, it is known to provide the latter with a metal wire which is visible in X-rays. For this purpose, a circumferential groove can be worked into the surface of the implant, and the radiographic contrast wire is inserted into this groove. To secure the wire on the implant, the two ends of the wire are usually twisted together. During use of the plastic implant, the wire can become stressed and break. If the wire then migrates from its intended position, it may be necessary to remove it by performing an operation.
To counter the danger of the wire breaking, instead of twisting the ends of the metal wire together, it is known to bend them off and push them into two bores which have been worked perpendicularly into the surface. It has also been attempted to provide one end of the wire with an eyelet and to hook the other end into this eyelet in such a way that a certain play remains. However, the results of these attempts have not been satisfactory.
It is therefore an object of the invention to make available a plastic implant of the type described, in which the danger of the radiographic contrast wire breaking and/or the danger of the radiographic contrast wire migrating out of position is/are avoided.
For this purpose, the invention provides that the channel receiving the radiographic contrast wire is completely or partially closed off from the outside.
In this way, the wire is held in the channel and is prevented from migrating outwards. Every spatial configuration preventing a wire held in the channel from migrating outwards is included here. The wire ends do not have to be connected to one another. The wire thus forms an open loop which yields with deformations of the implant and is not subjected to any appreciable stresses. For example, upon expansion of the implant, the wire can be adapted to the increased length of the implant circumference by means of sliding along the channel. This applies to the use of an open wire loop and also applies in the case of a break in a closed wire loop.
The term xe2x80x9coutwardsxe2x80x9d relates to the cross-sectional shape of the channel. The course of the channel is not limited to certain shapes. It generally follows the course of the edge. The channel does not have to be closed.
The channel can be closed substantially continuously along its entire length or along most of its length. However, it is generally not a problem if this closure is missing in individual sections of the channel, as long as the sufficiently secure holding of the wire therein is guaranteed. It may even be sufficient to have only a few closed areas distributed about the circumference.
The closure of the channel from the outside does not have to be complete in cross section; instead, partially closed channels are also included here by preference. An only partial closure of the channels from the outside is preferred with a view to permitting production by machine-cutting.
The partial closure of the channel from the outside is preferably obtained by a restriction of its profile, the width of said restriction being less than the wire thickness. To ensure that the wire is clearly positioned, its diameter is advantageously only slightly less than the channel diameter. The wire can be introduced into the channel by, for example, pressing it in from the outside through the restriction. For this purpose, in one possible embodiment, the implant is made elastic in the area of the restriction and the width of the restriction is also dimensioned in such a way that the restriction can be elastically widened for introducing the wire.
However, another embodiment is preferred in which the channel has an insertion opening along a circumference length sufficient for inserting the wire. On the one hand, the insertion opening must have a sufficient length (in the lengthways direction of the channel) to permit easy insertion of the wire into the channel. On the other hand, the insertion opening must not be too long, in order to prevent emergence of the wire in the event of lengthways movements of the wire in the channel. In this respect it is advantageous if both ends of the channel open out in the insertion opening, in particular in alignment with one another, so that a wire end migrating out of one channel opening passes back into the other channel orifice on the opposite side of the insertion opening. In this way, the wire stays trapped within the channel. If the channel is curved, it is advantageous if the wire, in the unstressed and unassembled state, has approximately the same curvature as the channel. If its end migrates out of one channel orifice, then it follows the continuation of this curved path, on which the other channel orifice also lies, so that it is all the easier for it to pass back into this other channel orifice. The term xe2x80x9calignmentxe2x80x9d in this context is to be understood as meaning that their curved axes coincide.
A sufficient securing of the ends of the wire in the withdrawal opening requires that a wire end migrating out of one channel orifice passes back into the other channel orifice, although this is not absolutely essential; it is instead sufficient if a wire end migrating out of the channel strikes against an opposite wall of the withdrawal opening, thus preventing further movement bringing the wire out of the channel.
Migration of the wire out of the insertion opening can also be prevented by the fact that the wire ends, which in their mounted position each protrude from the channel orifices, are bent away from the channel axis. Displacement of the wire is then limited by means of the contact of the bent ends against the associated channel ends or other walls formed by the insertion opening. In order to provide space for the bent ends, the insertion opening is advantageously widened in relation to the cross section of the channel.
The wires are preferably not bent at the channel ends but instead at a certain distance from said ends, as a result of which a certain play remains for compensating for the above-described changes in length.
In order to limit the friction between the wire and the outer wall of the channel during insertion, it is advantageous if the circumferential channel has a monotone curvature. Areas with a strong curvature are advantageously avoided. To ensure that the wire cannot become jammed in the restriction when being pushed into the channel, the width of the restriction is at most 80%, preferably at most 70% and still more preferably at most 60% of the diameter of the wire. The wire which is to be pushed into the channel is advantageously rounded, at least at its leading end, so that it can be pushed in more easily.
The present invention is particularly advantageous for use in intervertebral endoprostheses with a sliding core made of nonrigid plastic such as polyethylene, in which case it is possible that, as a result of pressure stresses, the sliding core will expand, with enlargement of its circumference.